1. Field of the Invention
The present invention relates to a detector, and a system utilizing the foreign-matter detector, for detecting the presence or absence of foreign matter in articles.
2. Description of the Prior Art
In general, the foreign-matter detector is known as an equipment for detecting the presence or absence of foreign matter such as, for example, metal in articles which ought not to contain such foreign matter. This foreign-matter detector is generally comprises a transmitter, a receiver, a signal extractor for extracting a received signal having a predetermined phase difference relative to a transmitted signal from the transmitter, based on the phase of the transmitted signal from the transmitter and the received signal from the receiver, while an article to be inspected is passed through the magnetic field generated by the transmitter, and a determining means for comparing an extracted signal from the signal extractor with a predetermined determination level and for determining the presence or absence of foreign matter in the article based on a result of the comparison.
In this foreign-matter detector, since an article free from foreign matter has an effect (a product effect) that affects the magnetic field, it often occur that such article reacts to the foreign-matter detector to eventually generate the extracted signal of a high level having the predetermined phase difference. For this reason, to avoid any possible erroneous detection caused by the article itself, it is necessary to set, for each type or kind of articles, the level of the extracted signal, which is attributable to the article itself, by passing, before the foreign-matter detector is actually operated, the article free from the foreign matter and adjusting the detection sensitivity of the receiver so that the detector will not react. In such case, to assuredly avoid the erroneous detection which would result from the product effect, setting of the extracted signal attributable to the article itself has been carried out by setting the detection sensitivity at the phase of such extracted signal to a relatively low value, that is, reducing the detection sensitivity, to attain a predetermined fixed value lower than the level of determination of the foreign matter. The difference between the level of the extracted signal attributable to the article itself and the determination level represents a sensitivity margin.
However, even though compositions of the articles to be inspected remain the same, the value of the extracted signal attributable to the article varies depending on the characteristics of the article, for example, change in shape and stability of the article during transportation thereof, and therefore, it has often occurred that the sensitivity margin referred to above has to be increased or decreased. By way of example, in the event that a change of the extracted signal attributable to the article itself is small, the level of the extracted signal can be set to a level higher than the above described fixed value to thereby decrease the sensitivity margin, since there is little possibility that the erroneous detection in which the article free from foreign matter is determined as containing foreign matter would occur. When the level of the extracted signal attributable to the article itself is increased, the sensitivity of detection of the foreign matter will increase, accompanied by a corresponding increase of the performance of the foreign-matter detector. In such case, it has been a general practice that the operator performs a job of manually changing the reading of a sensitivity setting indicator (for example, 0 to 999) of the foreign-matter detector to thereby adjust the detection sensitivity so that a desired extracted signal can be obtained.
With the prior art foreign-matter detector of the type discussed above, adjustment of the detection sensitivity is required frequently each time the reading of the detection sensitivity is changed accompanying passage of the article through the foreign-matter detector and, therefore, a relatively long time is required to set the extractor signal to a desired level. Also, it is often required to properly adjust the phase of the extract signal depending on the characteristics of the article.
Accordingly, the present invention has been devised with a view to substantially eliminating the above discussed problems inherent in the prior art foreign-matter detector and is intended to provide an improved foreign-matter detector and an improved foreign-matter detecting system in which the level of the extracted signal attributable to the article to be inspected can easily be set.
To this end, the present invention provides a foreign-matter detector for detecting foreign matter contained in an article transported between a transmitter and a receiver. This foreign-matter detector comprises a signal extractor operable, based on the phase of a transmitted signal from the transmitter and a received signal from the receiver, to extract the received signal having a predetermined phase difference relative to the transmitted signal, a specific value setting unit operable in response to an external input to set to a specific value the level of the extracted signal fed from the signal extractor in connection with an article free from foreign matter to be detected, and a level adjusting circuit for adjusting the signal extractor or the receiver to adjust the level of the extractor signal, attributable to the article itself, to the specific value when the article free from the foreign matter to be detected is transported.
With the foreign-matter detector according to the present invention, when the specific value for the level of the extracted signal attributable to a desired article itself is inputted, the signal extractor or the receiver can be automatically adjusted to assume the specified value for the article itself and, therefore, the setting of the level of the extracted signal attributable to the article can easily be accomplished.
Preferably, the specific value setting unit referred to above includes an item input unit for inputting kinds of articles, a storage means storing a specific value for the level of the extracted signal attributable to each kind of the articles, and a specific value selecting means for reading out the specific value corresponding to the kind of the article inputted and for outputting the specific value to the level adjusting circuit. Accordingly, merely by inputting the kind of the article, a receiving circuit of the receiver or a transmitting circuit of the transmitter can be automatically adjusted so that the level of the extracted signal attributable to the desired article itself can assume the specific value and, therefore, the setting of the level of the extracted signal attributable to the article itself can further easily be accomplished.
Again preferably, the level adjusting circuit is operable to perform at is least adjustment of the detection sensitivity, out from adjustment of the phase of the transmitted signal in the signal extractor and adjustment of the detection sensitivity in the receiver, to thereby adjust the level of the extracted signal attributable to the article itself. In such case, in a preferred embodiment of the present invention, the receiver comprises a pair of receiving coils connected parallel to each other and disposed at respective locations where they receive an equal amount of magnetic fluxes generated by the transmitter, and a differential amplifier for amplifying a difference between respective voltages generated in the receiving coils. The amplification factor of the differential amplifier can be adjusted by the level adjusting circuit.
The present invention also provides a foreign-matter detecting system which comprises a transport conveyor for transporting successively articles which may be a food product, and the foreign-matter detector of the type referred to above for detecting the presence or absence of foreign matter contained in the article being transported by the conveyor. In this system, the presence or absence of foreign matter in the article can be detected while it is transported by the conveyor.